One of the major problems facing industries such as mining, precious metals, and energy (e.g., coal mining and coal-fired power plants) is removal of toxic metal ions from process effluents. Stringent standards for the maximum level of pollutants in water that is funneled into domestic and ground water systems are being promulgated by federal and state agencies. Thus, there is a need in the art for simple and effective methods for the removal of such metals from process effluents.
Polymerization of metal alkoxides in the presence of molecular assemblies of surfactants or related substances, acting as structure directors, has resulted in several novel classes of mesoporous and macroporous inorganic materials with extremely high surface areas and ordered mesostructure. The mechanism for the organization of such mesostructure involves electrostatic interactions and charge matching between micellar assemblies of quaternary ammonium cations and anionic silicate oligomer species. These materials have now found extensive applications as catalyst supports and chromatographic resins.
Many applications of mesoporous materials require functionalization of the material's silanol surfaces. Presently, extensive research is being conducted to develop procedures to introduce functional silane ligands into the surfaces of the ordered mesoporous materials
Feng et. al. (Science: 276, 923-926; 1997) and Mercier and Pinnavaia (Advanced Materials: 9, 500-503; 1997) have developed new, effective mesoporous sorbents for the removal of toxic metal ions based on mesoporous materials as supports. Their methodology involves coating surfaces of hexagonally packed mesoporous silica with organic functional groups to enhance their affinities for metal ions. High capacities and fast kinetics have been observed for these new sorbents. The selectivity of these materials relies solely on the affinity of the surface-coated functional ligand for a specific metal ion, with no consideration of the stereochemical interactions between the ligand and the metal ion. Thus, there is a need in the art for mesoporous sorbents having stereochemical specificity.
Bulk molecular imprinting methods based on the template approach have been used in crosslinked polymers, as well as in silica gels, to prepare polymeric supports possessing solid-state organized structures. Imprinting processes generally involve three steps: (1) selection of a target molecule as a template; (2) incorporation of the template into rigid solid networks through in situ copolymerization; and (3) removal of the template, leaving cavities with a predetermined number and arrangement of ligands that later "recognize" or selectively rebind the template or target molecule. These imprinted organic polymers have been used to resolve racemates and separate mixtures of metal cations. One major drawback associated with this bulk molecular imprinting technique is that the kinetics of the sorption-desorption process are unfavorable, as the template and ligand are totally embedded in the bulk polymer matrices and the mass transfer must take place through nonpolar, microporous channels. Furthermore, molecular imprinting studies have thus far all been conducted in disordered polymers or amorphous sol-gel matrices where the inhomogeneity of the cavities produced by the molecular imprinting reduces the selectivity of the final imprinted materials. Thus, there is a need for a bulk molecular imprinting technique that provides favorable sorption-desorption kinetics.
The generally accepted mechanism for such direct coating methods involves the initial hydrolysis of siloxane groups in the functional silane ligand followed by condensation with the surface silanol groups to produce ligands that are covalently bound or "tethered" to the surfaces. Drawbacks associated with this coating method are the time-consuming reflux syntheses and low loading of the functional silane ligands. Thus, a simpler and more efficient method for imprint coating is needed.